Known are calibration methods, which consider the type of disturbance, the distance to the disturbance and knowledge of how to reach a completely developed flow profile. A corresponding method is disclosed in US 2009/055119 A1. As a result of this, a flow measuring device is provided, which is preconditioned for a certain flow situation in the plant. This means that this flow measuring device is tuned for a certain flow situation and delivers in this situation a highly accurate measurement. As soon as the flow situation changes, thus the disturbance type and/or the distance to the disturbance, then the flow profile changes, so that the measuring device no longer works correctly. Therefore, the device is only applicable for the situation, for which it was calibrated in the plant.
German Patent DE 197 17 940 A1 describes the correction of a vortex intensity. Besides the vortex intensity, also a ratio between longitudinal impulse and rotational impulse in the medium is taken into consideration. A flow profile asymmetry is, in such case, not corrected. Thus, this correction method is limited to certain flow profile disturbances, namely vortices.
US 2003/0131667 A1 describes a method for compensating flow profile asymmetries and vortices with the assistance of flow conditioning systems. The flow conditioning systems e.g. tube bundle flow straighteners and/or perforated plates assure that defined flow conditions reign at the location of the ultrasonic flow measuring. In this way, a defined and exact flow measuring is enabled, however, while having to accept in the bargain high pressure losses and obstruction of the flow cross section. In this situation, e.g. a cleaning of the pipeline by means of a pig is impossible. Due to the pressure losses, also a significantly higher pumping power is required. The user is compelled, especially in comparison to clamp-on flow measuring devices, to open the pipeline, in order to install the flow conditioner.
U.S. Pat. No. 5,987,997 A describes a method for Reynolds number ascertainment by comparing flow velocities along five different measuring paths and the correction of the flow profile based on the ascertained values. Especially, these measuring paths must be distributed in a certain manner across the measuring cross section, since a different radial separation of the measuring paths is required. This method rests on ascertaining the velocity ratios of these measuring paths relative to one another. This is essential in the case of this method.
The intermediately published WO 2013/164805 A1 discloses an ultrasonic, flow measuring method, which corrects for flow disturbances and which requires the flow type, the position and the orientation specifications of the transducer pair. Thus, a high calibration effort is required, since the correction is composed of three independent parameters. Especially the orientation specifications in the case of a single path system are essential, in order to be able to perform a reliable correction. Even small deviations can lead here to a large error. Therefore, individual measurements with very small angle change, respectively step width, are required, in order to enable a sufficiently exact and flexible correction. In some cases it is not per se possible for the user to give an exact orientation specification for a disturbance, since the disturbance can be of complex nature or is unknown and the positioning of the transducer relative to the disturbance can, consequently, not be given exactly.
Clamp-on ultrasonic, flow measuring devices are known, which work using a so-called two path arrangement. This measuring occurs via two or more ultrasonic transducer pairs. This arrangement has the advantage that measurement inaccuracies in the case of not completely symmetric, especially not completely rotationally symmetric, flow profiles are partially compensatable. Sources for such disturbances are variable cross sections, branches, flow deflections and components, which are connected to a pipeline and those are considered as a component of a pipe, respectively as a pipe element. These include, among other things, elbows, valves or even pumps.
Depending on the type of the disturbance, a certain minimum separation is recommended for the arrangement of the ultrasonic transducers around the pipe, since otherwise a device-specific upper limit for the accuracy of measurement can no longer be assured.
There are, however, applications, in which this minimum separation, the so-called run-in distance, can not be maintained. In the case of such applications, large measurement errors occur as a result of the not completely developed flow profile. Here, however, the customer has also no information relative to the accuracy and reliability of the measuring, with which it can estimate and evaluate the quality of the ascertained information.